Ventilation systems and air conditioning systems commonly use a fan comprising a fan blade or impeller driven by an electric motor to create the air flow and adjust the air flow by changing the speed of the motor. However, such speed changes are usually a 2 or 3 step change, manually selected by an operator. When the static pressure of the system varies, which may be caused by changes in the ductwork of the system, in exterior wind pressure or the like, the volume of air moved by the system also varies. However, for certain installations it is desired to keep the volume of air flowing through the system constant, regardless of static pressure variations.
The present invention aims to provide a new method to keep the volume of the air flow constant.